The invention relates to rotary drill bits for use in drilling or coring deep holes in subsurface formations.
The invention is applicable to rotary drill bits of the kind comprising a bit body having a shank for connection to a drill string, a bit face on the bit body, a plurality of cutting structures mounted in sockets in the bit body and projecting from the face of the bit, and a number of nozzles also mounted in sockets in the bit body and communicating with a passage for supplying drilling fluid to the face of the bit, at least part of the bit body being formed from steel.
Each cutting structure may comprise a cutting element mounted on a carrier, such as a stud or post, which is received in a socket in the bit body. One common form of cutting element comprises a circular tablet having a hard facing layer of polycrystalline diamond or other superhard material and a backing layer of less hard material such as cemented tungsten carbide. The two-layer arrangement of the cutting element provides a degree of self-sharpening since, in use, the less hard backing layer wears away more easily than the harder cutting layer.
In such steel-bodied drill bits the bit face is susceptible to wear and erosion during use, particularly in the vicinity of the nozzles from which driling fluid emerges at high velocity and with substantial turbulence. Accordingly, attempts have been made to apply a hardfacing to the bit face, around the cutting structure. U.S. Pat. No. 4,396,077 describes, for example, a method of applying a tungsten carbide hardfacing to a steel bodied drill bit after the cutting structures have been assembled in the sockets.
However, as discussed in U.S. Pat. No. 4,396,077, the application of a hardfacing to the bit body after the cutting structures have been assembled into it carries with it the risk of thermal damage to the cutting elements during the hardfacing process. Accordingly, it is desirable to provide the hardfacing before the cutting structures are assembled with the bit body. However, this in turn gives rise to other problems, particularly with regard to preventing the hardfacing material entering the sockets. In order to prevent coating material from entering the sockets, it has been the practice to plug the sockets temporarily with plugs of graphite or mild steel. However, there is then a conflict between the requirement that the formers should tightly fill the sockets to prevent any possibility of ingress of coating material and the requirement that the formers have to be removed from the sockets after the hardfacing process. Using the former materials employed hitherto, if the formers are sized to fit sufficiently tightly within the sockets to prevent any possibility of ingress of the hardfacing material, then the formers must be removed destructively, for example by drilling them out. Not only is this a time-consuming process, but it may also mean that the internal surfaces of the sockets become damaged during removal of the formers with the result that they have to be reamed to the correct size. On the other hand, if the formers are sized so that they may be more readily removed from the sockets after hardfacing, there is an increased risk of ingress of coating material into the sockets so that, again, the sockets may have to be accurately reamed to size. The present invention provides an improved method of manufacturing a rotary drill bit where the above problems may be reduced or overcome.